This invention relates to a directly heated type oxide cathode.
As cathodes used in television display tubes and the like, there are an indirectly heated type oxide cathode and a directly heated type oxide cathode. Since the directly heated type oxide cathode requires about 1 to 2 seconds from the beginning of passing the current to the appearance of images and is superior to the indirectly heated type oxide cathode (about 20 seconds) from the viewpoint of saving energy, it is widely used now.
The directly heated type oxide cathode in principle has a structure as shown in FIG. 1, wherein numeral 1 is a base made of an alloy containing Ni as a major component, numeral 2 is a terminal and numeral 3 is an electron emissive alkaline earth oxide layer (hereinafter referred to as "oxide layer"). Heretofore, in order to strengthen the bonding strength between the oxide layer 3 and the base 1, there has been employed a method for interposing Ni powders between the oxide layer and the base. But according to this method, there often takes place thermal deformation of the cathode during the production procedures of the cathode or during the use of the cathode after installing it, for example in an electron tube, which results in finally causing peeling of the oxide layer 3 from the base 1. The peeling of the oxide layer 3 from the base 1 means a stop of the action as cathode. Thus, various proposals have been made in order to prevent the peeling of the oxide layer.
One of main technical ideas among these proposals is, to say simply, that a main cause of the peeling of the oxide layer is thermal deformation of the base itself and the peeling of the oxide layer can be prevented if the thermal deformation of the base is prevented.
Based on such an idea, U.S. Pat. No. 4,081,713 discloses a directly heated type oxide cathode wherein both the upper and lower surfaces of the base is coated with Ni and the like powders and the oxide layer is formed on the upper surface of the base. Such a structure is based on an idea that diffusion of the Ni powders proceeds on the both surfaces with the evolution of heat from the base and stress applied to the base can be balanced on the both surfaces, which results in minimizing the deformation of the base, preventing the peeling of the oxide layer and further preventing at the same time an undesirable phenomenon of the loss of a white balance shown in the three electron guns for a color cathode-ray tube.
But even if employing such a structure, it was found that the deformation of the base cannot be prevented completely. There is disclosed in Japanese Patent Application Kokai (Laid-Open) No. 62454/78 a directly heated type oxide cathode wherein a dense layer of Ni and the like is additionally formed between the base and the Ni powders, or the dense layer of Ni and the like is formed on the lower surface of the base in place of the Ni powders in addition to the Ni dense layer between the upper surface of the base and the Ni powders. Such a structure is based on an idea that strain produced in the base can be suppressed by diffusion of Ni on the both surfaces of the base with the progress of evolution of heat from the base, and the dense Ni layer can prevent influence of the oxide layer, particularly influence during the step of thermal decomposition of the carbonates, which results in minimizing the deformation of the base and thus preventing the peeling of the oxide layer.
A common point of the above-mentioned two proposals is to prevent the deformation of the base by forming layers of Ni and the like on the both surfaces of the base taking the diffusion of Ni into consideration. Therefore, as to Co which is chemically analogous to Ni, the behavior of Co is regarded as the same as that of Ni and there is described that the same results as obtained in the case of Ni can be obtained if a part or whole or Ni is replaced by Co.
But the coating of Ni powders or the formation of Ni layers on the both surfaces of the base not only makes the production procedures complicated but also gives disadvantages in that high technique is required for providing an equivalent amount of Ni to the both surfaces, and if Ni is provided ununiformly on the both surfaces, the deformation of the base is rather accelerated. Further, the formation of uneven layer of Ni on the lower surface of the base gives another defect in that minute adjustment of the gap cannot be expected when the cathode is installed in an electron tube and the like.
On the other hand, there have also been made proposals for forming a layer of alloy containing Ni as a major component on only the upper surface of the base, i.e. the surface on which the oxide layer is formed, not on the both surfaces of the base. For example, the structure as shown in FIG. 2 is disclosed in U.S. Pat. No. 4,129,801. According to said U.S. patent, a metal layer 6 made of an alloy containing at least one of W and Mo in an amount of 10% by weight or less, Zr 1.5% by weight or less and the balance at least one of Ni and Co is formed on the base 1, and a bonding layer 7 is formed on the metal layer 6, said bonding layer being made of powders of Ni-Co alloy or a mixture of Ni and Co powders (Co 35-65% by weight and Ni 65-35% by weight), and an oxide layer 3 is formed on the bonding layer. The feature of this structure is to have the bonding layer 7 made of the special Ni-Co alloy and the metal layer 6 may be omitted. Said U.S. patent is based on an idea that the deformation of the base can be prevented by applying the fact that unbalanced diffusion between Ni powders-to-the base and Co powders-to-the base is reversed, said unbalanced diffusion causing the deformation of the base, that is, by offsetting deformations due to the mutual diffusion by simultaneous use of Ni and Co having mutually reversed actions to the thermal deformation of the cathode base. In such a case, the powders of Ni-Co alloy has the same action as that of the powder mixture of Ni and Co. Therefore, it is necessary to bake the Ni powders and Co powders to the same extent. But since uniform coating of the Ni and Co powders is very difficult and baking properties of the Ni powders, Co powders and the base metal are different, the baking procedure is very difficult. Further, since unbalance in diffusion is dependent on the composition of the base, it is necessary to change the mixing ratio of the Ni powders and the Co powders depending on the composition of the base. In addition, according to said U.S. patent, when Ni and Co powders having almost the same particle size or Co powders having a larger particle size than Ni powders are used, the baking of Co is preceded by the baking of Ni and the reaction between Zr and the oxides is not suppressed, so that the deformation of the Ni powders takes place and thus the deformation of the base cannot be prevented. Further, there is a tendency to easily bring about the peeling of the oxide layer and to increase the degree of deformation itself with an increase of the Zr content in the base.
As to alloys for forming the base, there have been known those containing Ni as a major component, one or more high-melting point metals such as W, Mo, etc. in order to increase heat resistance and mechanical strength, and a reducing agent, i.e. a metal or element which functions as an activating agent for the electron emissive oxides, such as Mg, Al, Si, Zr, or the like. The use of Zr as reducing agent is preferable on maintaining activity of the cathode for a long period of time as well as from the viewpoint of cold workability of the base. But when the base is made of an alloy containing Zr as reducing agent, Ni powders or Ni dense layer is coated on the base in order to prevent the peeling of the oxide layer according to the conventional method and the electron emissive oxide layer is formed thereon, there is a tendency that the oxide layer is easily peeled off during the operation of an electron tube in which the resulting cathode is installed. Causes of such a phenomenon have not fully been understood from the conventional viewpoints of diffusion of Ni and Co, diffusion of W, and the like.